This invention relates to a heated galvanic solid electrolyte oxygen sensor, and more particularly to improvements in a heater-reference electrode terminal subassembly for such a sensor.
Solid electrolyte galvanic oxygen sensors essentially include an oxygen ion conductive ceramic body with porous electrodes on opposite faces of the body. One electrode is exposed to a reference source of oxygen. The other electrode is exposed to a source whose oxygen content is to be measured. A difference in oxygen partial pressure at the electrodes produces a corresponding potential difference across the electrodes, which provides a sensor output voltage.
The output voltage of such sensors can be used as a measure of oxygen or unburned combustibles in exhaust gases from an internal combustion engine. This voltage can be used in both monitoring and controlling the engine combustion process, as disclosed in U.S. Pat. Nos. 3,616,274 Eddy, 3,844,920 Burgett et al. and U.S. Ser. No. 787,900 Howarth, filed Apr. 15, 1977, now U.S. Pat. No. 4,129,099, and assigned to the assignee of this invention. To obtain an appreciable output voltage, the sensor solid electrolyte is heated to an elevated temperature. Also, sensor output voltage varies with changes in temperature, especially at lower operating temperatures. Combustion gases can be used to heat the sensor to operating temperatures but such gases vary widely in temperature, particularly when from an internal combustion engine. Moreover, the combustion gases may cool significantly before contacting the sensor or may not heat the sensor fast enough on start up. Consequently, it has previously been proposed to supply supplementary heat for the sensor, and even include a resistance heater in the sensor itself. One such heated sensor construction is disclosed in the United States patent application Ser. No. 892,642, filed by Murphy et al. on Apr. 3, 1978 and entitled "Solid Electrolyte Oxygen Sensor with Electrically Isolated Heater." Murphy et al. disclose a sheathed heater insulatingly supported in subassembly with a tubular terminal for a reference electrode on the solid electrolyte of the sensor. A resistance heating coil is buried within a ceramic powder contained within the heater sheath. The heater is aligned with the electrode terminal. When the electrode terminal is aligned with the solid electrolyte body, the heater is inherently also aligned with it. A ceramic sleeve spaces the heater and electrode terminal and bonded to them in subassembly by a fused glass. This provides a rugged, reliable and readily manufacturable and readily assembled subassembly for a heated automobile exhaust gas oxygen sensor in which all terminals are coaxial, and the heater is electrically isolated from sensor terminals. in this way the aforementioned U.S. Ser. No. 892,642 presents an improvement on the heater-terminal subassembly concept claimed in the U.S. patent application Ser. No. 892,644 Murphy that was concurrently filed therewith and entitled "Heated Solid Electrolyte Oxygen Sensor."
We have now found how to improve the isolated heater-electrode terminal subassembly even further. We have found how to increase heating efficiency, reduce manufacturing costs, and even enhance bonding of the heater-terminal subassembly. Moreover, yields in manufacturing of the heater in this subassembly should be higher. The manufacturing and assembly techniques are more familiar, and do not require as much precision in performance.